Chromium is an important constituent of a wide variety of alloys, including various steels as well as nickel-based, cobalt-based, and copper-based alloys. Chromite ore from which the metal is derived is found in abundance in relatively few places, notably in southern Africa, whereas large amounts of chromium-containing scrap are generated in most industrialized countries and recovery of the chromium therefrom is highly desirable.
The recycling of chromium-containing scrap metal does not pose any insurmountable difficulties in the case of stainless steel production. This is due in part to the ability to refine in the melting vessel as in the case of the AOD process. However, the same is not true in the case of production of many complex alloys which may contain five or even ten alloying elements. Such alloys are often very sensitive to contamination, and are produced by processes which do not lend themselves to any extensive refining in the melting vessel. As a result, producers of complex alloys, such as the so-called superalloys, tend to rely on pure metals for their feed, while chromium-containing scrap is downgraded to produce less demanding alloys.
While procedures have been proposed in the past for separating the alloy constituents of scrap metals, none of the procedures to our knowledge has ever reached the stage of commercial implementation. This is undoubtedly ascribable to the cost and complexity of such procedures. Thus, methods of treating superalloy scrap are described in U.S. Pat. No. 3,544,309 (to A. W. Fletcher et al) as well as in the publication by P. T. Brooks et al entitled "Chemical Reclaiming of Superalloy Scrap," U.S. Department of the Interior, Bureau of Mines, 1969. The methods are directed primarily at recovering nickel and cobalt from the scrap and entail a complete dissolution of the scrap, after which the various metals are separated by hydrometallurgical steps.
A process which avoids the slow and indiscriminate procedure of putting the whole of the scrap into solution is described in a published Japanese patent application, No. 73-44121 by T. Goto. The process described therein involves an initial pyrometallurgical treatment in which superalloy scrap is melted and blown with oxygen until such metals as aluminum, titanium, and silicon as well as much of the chromium content of the melt have been oxidized. Sulfur is then added and the bath is reblown to remove iron and chromium. The result is that iron and chromium are removed together in a slag high in titanium, aluminum, and silicon, while a matte said to contain 60.8% Ni, 14.2% Co, 0.4% Cu, and 24.5% S is derived after the blowing. The separate recovery of chromium or iron from the resulting slag is unlikely to be economically feasible.
Also worthy of mention in the present context in U.S. Patent No. 3,313,601 (to O. F. Marvin) despite the fact that it does not relate to the treatment of scrap. The Marvin patent is concerned with the treatment of complex oxide ores, and describes an example wherein a chromite ore concentrate is heated to about 870.degree. C. in the presence of CS gas whereby it is said to be converted to a mixture of chromium sulfide, iron sulfide, and iron oxide. The cooled mass is subsequently leached to leave a residue of chromium sulfide. The outcome of attempting such a solid state sulfidation on alloy scrap is unknown, and the subsequent hydrometallurgical separation process would be much more complex in a multicomponent system.